The relative degree of thermoplasticity of acetylated cotton yarns has been determined by a technique developed for the purpose. The interfering contribution of hydrogen bonding could be eliminated by a wetting-out process. The yarns were acetylated to degrees of substitutions ranging from 0.77 to 2.93. Heat setting was applied either in vacuum or at atmospheric pressure in the presence of water vapor at temperatures in the range of 110–225°C. The thermoplasticity observed over a period of 30 days was permanent but readily removable by another application of heat setting. The effect of degree of substitution upon thermoplasticity could be represented by sigmoidal curves beginning at about DS = 1 and increasing rapidly and asymptotically to a maximum at about DS = 2.25. In vacuum or water vapor the maximum thermoplastic response occurred at 175°C., but the response was greater with water vapor. Under vacuum heating a very distinct minimum response occurs at 180°C., due to second-order transition effect. Upon heating in the presence of water vapor a distinct minimum response occurs at 200°C. with secondary minima at 150 and 120°C. Prior crystallization (by heating to 200–225°C.) greatly reduces thermoplasticity due to reduction of the amorphous component, but does not completely eliminate it.